1. Field of the Invention
The present invention relates to a compact spark gap switch capable of generating pulse voltage of high voltage and high frequency for a long time, in which the pulse width and the peak value are nearly uniform and to a switching method thereof, particularly to the spark gap switch, which is suitable for using as a high voltage pulse generator such as a plasma generator utilizing corona discharge and to the switching method thereof.
2. Description of the Prior Art
Generally, in a plasma generator for generating plasma in air by means of corona discharge, the plasma is generated by applying pulse voltage of high voltage and high frequency between a discharging electrode and an opposite electrode, both of which are disposed to face to each other, so as to cause corona discharge between the both electrodes. Thus, in order to generate the pulse voltage of high voltage and high frequency as described above, generally, there has been used an impulse circuit using a spark gap switch.
In the impulse circuit described above, generally, the pulse voltage of high voltage and high frequency is generated by repeating such processes of providing (charging) electric charge to a charge and discharge capacitor by means of a DC electrical source of high voltage, conducting the circuit by means of the spark gap switch so as to discharge the electric charge of the charge and discharge capacitor, and generating the pulse voltage of a pattern decided in accordance with resistance, inductance and capacitance of the circuit. Hereupon, the spark gap switch, in which a first spherical electrode connected to a unipolar output terminal (positive or negative) of the DC electrical source and a second spherical electrode connected to a positive output terminal are disposed so as to face to each other while holding a predetermined spark gap (interval) therebetween, becomes a conducted state (ON) when the spark discharge is caused between the both spherical electrodes. On the other hand, it becomes an intercepted state (OFF) when the spark discharge is not caused between the both electrodes. Therefore, in the impulse circuit, the frequency of occurrence of the pulse voltage coincides with the frequency of occurrence of the spark discharge in the spark gap switch.
In the spark gap switch described above, the larger the spark gap between the both spherical electrodes which are disposed to face to each other becomes, the higher charging voltage required for causing the spark discharge becomes, in consequence the pulse voltage becomes higher. However, if the spark gap is larger, it becomes harder to satisfy the conditions required for starting the spark discharge. Accordingly, there occurs such a problem that control of the spark discharge becomes more difficult so that the pulse voltage may not be stabilized. On the contrary, if the spark gap is smaller, there may be obtained such an advantage that the control of the spark discharge becomes easier and the frequency of occurrence of the spark discharge increases so that the pulse voltage may be stabilized. However, there may be occur such a problem that the pulse voltage is not raised so much, because the charging voltage required for causing the spark discharge is lowered. That is, in the conventional spark gap switch, there exists such a problem that it is difficult to stabilize the pulse voltage without lowering the pulse voltage.
Further, in the impulse circuit using the spark gap switch described above, there may be caused such a problem that the charging circumstance of the spark gap is deteriorated when the pulse voltage is generated for a long time. That is, if remaining ions, fine metal particles (fine metal chips) and so on stay in the spark gap after a certain spark discharge has been completed, the following spark charge may be started at voltage lower than that of the preceding spark discharge. Accordingly, the resultant pulse voltage may become lower, and further it may become harder to obtain uniform pulse voltage
Thus, the applicant (inventor) of the present application has previously proposed a spark gap switch in which the position, where the spark discharge is caused, is fixed (specified) by providing respective protruding portions (protruding electrodes) on the respective positions of the both spherical electrodes, the both positions being disposed so as to face to each other (Japanese Laid-Open Patent Publication No. 7-235362). Further, in the spark gap switch, air is let flow through the spark gap, thereby remaining ions and fine metal particles are prevented from staying there so that the stability (uniformity) and durability of the pulse voltage may be raised.
However, in the above-mentioned conventional spark gap switch according to the applicant of the present application, the following problems are still remained. That is, if the spark gap is enlarged in order to achieve much higher voltage, it is required to delicately (precisely) adjust the length of the spark gap or the dimension of each of the protruding portions (protruding height) or to precisely control the flow rate of the air fed to the spark gap in order to achieve stable pulse voltage. Further, there exists also such a problem that regions of various working conditions, in which the switch can work stably, may become narrower. Accordingly, there may occur such a problem that it is difficult to successively generate the pulse voltage of high voltage and high frequency for a long time.
For example, in order to generate higher voltage pulse, it is required that the length of the spark gap is longer than 150 mm. However, when the spark discharge is caused with the spark gap described above, it is very difficult to adjust the protruding height of each of the protruding portions, the length of the spark gap and the condition for feeding air. Therefore, in order to achieve stable pulse voltage for a long time, delicate adjustment is required and further the working condition, in which the switch can maintain stable working, becomes narrower. Further, because the switch tends to be affected by the temperature, it is indispensable to precisely control the spark gap switch in order to let the switch work stably for a long time. Moreover, there exists also such a problem that the diameter of each of the spherical electrodes and the length of the spark gap are required to be longer in order to achieve the high pulse voltage so that the apparatus becomes large size.
The present invention, which is achieved to solve the above-mentioned conventional problems, has an object to provide a compact spark gap switch or a switching method thereof, in which pulse voltage of high voltage and high frequency can be generated stably for a long time and further control of spark discharge is easy.
According to the present invention, which has been achieved to solve the above-mentioned problems, there is provided a spark gap switch for performing a switching operation including first and second spherical electrodes disposed so as to hold a space therebetween, between both of which DC voltage is applied, and an intermediate electrode (third electrode) which is disposed at a nearly middle position between the both spherical electrodes in a spherical electrode aligning direction so as to hold respective spark gaps between the respective spherical electrodes and the intermediate electrode. Hereupon, the intermediate electrode is not connected to any electrical source. In the spark gap switch, the switching operation (change of ON/OFF) is performed between the both spherical electrodes by generating respective spark discharges in the spark gap (hereinafter, referred to xe2x80x9cfirst spark gapxe2x80x9d) between the first spherical electrode and the intermediate electrode and in the spark gap (hereinafter, referred to xe2x80x9csecond spark gapxe2x80x9d) between the intermediate electrode and the second spherical electrode.
In the spark gap switch, when high DC voltage (hereinafter, referred to xe2x80x9cinter spherical electrode voltagexe2x80x9d) is applied between the first spherical electrode and the second spherical electrode, there is caused such two-stage spark discharge that at first spark discharge is caused in any one of the first and second spark gaps, and then further spark discharge is caused in the other spark gap. The above-mentioned two-stage spark discharge, is caused with comparatively higher voltage, and further caused by nearly constant inter spherical electrode voltage. Therefore, the pulse voltage, which has nearly uniform output value (output voltage), may be stably generated for a long time. Further, the control of the spark discharge may become extremely easy. Moreover, the spark gap switch may be made compact, because the spark discharge is generated in high voltage and high frequency condition even if the spherical electrodes are comparatively small and the interval between the both spherical electrodes is comparatively short.
In the above-mentioned spark gap switch, it is preferable that the switch further includes a ventilator (for example, of 0-80 m/min flow velocity, or of 0-70 Nm3/min volumetric flow rate) for letting air flow through the space between the both spherical electrodes, in a direction nearly perpendicular to (or crossed with) the spherical electrode aligning direction. Hereupon, the intermediate electrode is preferably disposed at a downstream position in a flowing direction of the air, relative to a straight line linking the centers of the both spherical electrodes to each other. If so, the output value of the pulse voltage may be further made uniform, because remaining ions and fine metal particles do not stay in the space between the both spherical electrodes.
In the above-mentioned spark gap switch, it is also preferable that the intermediate electrode is formed in a spherical shape, a nearly cylindrical shape whose apex end has a hemispherical shape or a nearly flat plate shape whose apex end has a hemicylindrical shape. If so, the output value of the pulse voltage may be made uniform much further. In the spark gap switch, if the intermediate electrode is formed in a spherical shape, it is preferable that the first and second spherical electrodes are provided with respective protruding portions (trigger electrodes) for giving rise to respective spark discharges at such a position of the first spherical electrode that an interval between the first spherical electrode and the intermediate electrode is the shortest and such a position of the second spherical electrode that an interval between the second spherical electrode and the intermediate electrode is the shortest. Hereupon, the intermediate electrode is preferably provided with respective protruding portions (trigger electrodes) for giving rise to the respective spark discharges at such a position thereof that the interval between the first spherical electrode and the intermediate electrode is the shortest and such another position thereof that the interval between the second spherical electrode and the intermediate electrode is the shortest. If so, because the spark discharge is caused between the protruding portions which are disposed to face to each other, the positions, where the spark discharges are caused, are made constant so that the output value of the pulse voltage is made uniform much more. Hereupon, the protruding height of each of the protruding portions may be set to, for example, {fraction (1/100-1/8)} of the diameter of the corresponding spherical electrode. Further, the outer diameter (width) of each of the protruding portions may be set to, for example, {fraction (1/100-1/8)} of the diameter of the corresponding spherical electrode.
According to the present invention, there is also provided a switching method of a spark gap switch for performing a switching operation between a first spherical electrode and a second spherical electrode by means of spark discharge, by applying DC voltage between the both spherical electrodes, the both spherical electrodes being disposed so as to hold a space therebetween. Hereupon the switching method includes the step of disposing an intermediate electrode, which is not connected to any electrical source, at a nearly middle position between the both spherical electrodes in a spherical electrode aligning direction so as to hold respective spark gaps between the respective spherical electrodes and the intermediate electrode. Further, the switching method includes the step of generating respective spark discharges in the spark gap (first spark gap) between the first spherical electrode and the intermediate electrode and in the spark gap (second spark gap) between the intermediate electrode and the second spherical electrode so as to perform the switching operation between the both spherical electrodes.
In the switching method of the spark gap switch, when high inter spherical electrode voltage is applied between the first spherical electrode and the second spherical electrode, there is caused such two-stage spark discharge that at first spark discharge is caused in any one of the first and second spark gaps, and then further spark discharge is caused in the other spark gap. The above-mentioned two-stage spark discharge is caused with comparatively higher voltage, and further caused by nearly constant inter spherical electrode voltage. Therefore, the pulse voltage, which has nearly uniform output value (output voltage), may be stably generated for a long time. Further, the control of the spark discharge may become extremely easy. Moreover, the spark gap switch may be made compact, because the spark discharge is generated in high voltage and high frequency condition even if the spherical electrodes are comparatively small and the interval between the both spherical electrodes is comparatively short.
In the above-mentioned switching method of the spark gap switch, it is preferable to let: air flow through the space between the both spherical electrodes, in a direction nearly perpendicular to the spherical electrode aligning direction (for example, with 0-80 m/min flow velocity, or with 0-70 Nm3/min volumetric flow rate). Hereupon, it is preferable to dispose the intermediate electrode at a downstream position in a flowing direction of the air, relative to a straight line linking the centers of the both spherical electrodes to each other. If so, the output value of the pulse voltage may be made uniform further, because remaining ions and fine metal particles do not stay in the space between the both spherical electrodes.
In the above-mentioned switching method of the spark gap switch, it is also preferable to form the intermediate electrode in a spherical shape, a nearly cylindrical shape whose apex end has a hemispherical shape or a nearly flat plate shape whose apex end has a hemicylindrical shape. If so, the output value of the pulse voltage may be made uniform much further. Hereupon, if the intermediate electrode is formed in a spherical shape, it is preferable to provide the first and second spherical electrodes with respective protruding portions (trigger electrodes) for giving rise to respective spark discharges at such a position of the first spherical electrode that an interval between the first spherical electrode and the intermediate electrode is the shortest and such a position of the second spherical electrode that an interval between the second spherical electrode and the intermediate electrode is the shortest. Further, it is preferable to provide the intermediate electrode with respective protruding portions (trigger electrodes) for giving rise to the respective spark discharges at such a position thereof that the interval between the first spherical electrode and the intermediate electrode is the shortest and such another position thereof that the interval between the second spherical electrode and the intermediate electrode is the shortest. If so, because the spark discharge is caused between the protruding portions which are disposed to face to each other, the positions, where the spark discharges are caused, are made constant so that the output value of the pulse voltage is made uniform much more. Hereupon, the protruding height of each of the protruding portions may be set to, for example, {fraction (1/100-1/8)} of the diameter of the corresponding spherical electrode. Further, the outer diameter (width) of each of the protruding portions may be set to, for example, {fraction (1/100-1/8)} of the diameter of the corresponding spherical electrode.